In recent years, a hybrid vehicle having both an internal combustion engine and an electric motor as a power source has been developed. In general, there are two types of hybrid modes for a hybrid vehicle, i.e. a series mode and a parallel mode. In the series mode, a power generator is driven by power of the internal combustion engine, and an electric motor coupled with a drive axle is driven by the power obtained by the power generator, whereby the vehicle is made to travel. In the parallel mode, the drive axle is driven by the power of the internal combustion engine, the electric motor coupled with the drive axle is driven by the power obtained by a battery, and the vehicle is made to travel by using either one or both of the power of the internal combustion engine and the power of the electric motor in accordance with an operation state of the vehicle.
In the series mode as described above, the internal combustion engine is used only for driving of the power generator. Since the internal combustion engine can be operated in the most efficient operation condition of the internal combustion engine regardless of an operation state of a vehicle, fuel efficiency can be improved. In the parallel mode, the drive axle is driven by power outputted from the internal combustion engine and the power of the internal combustion engine is not converted to electric power in the power generator and there is no loss caused by conversion to electric power in the power generator, and thus, fuel efficiency can be further improved than the series mode.
However, in the series mode, the power is generated only by the electric motor, and it is known that an output torque decreases on a high rotation speed region due to characteristics of the electric motor.
In view of the above, a technology for ensuring a power performance is developed without deteriorating fuel efficiency of a vehicle by using the series mode in which travel is powered only by the electric motor at a low vehicle speed of the vehicle and by using the parallel mode in which travel is powered by the internal combustion engine and the electric motor when the vehicle speed rises (see Patent Literature 1).